Linear actuators mainly use electric motors and actuating elements, such as worm gears and worm wheels, to actuate leadscrews rotating. Meanwhile, the leadscrews actuate telescopic tubes screwed thereof extending out or retracting. Since the structure and the assembly are simpler and more convenient than pneumatic cylinders or hydraulic cylinders, the linear actuators are widely used in apparatuses, such as sickbeds, electric chairs and other fields that need expansion and retraction.
Conventional linear actuators are disclosed in EP0662573B and WO2004-027290A2, wherein EP0662573B discloses that an actuator mainly comprises a worm gear, a leadscrew, and a worm wheel assembly fixed at one end of the leadscrew. The worm wheel assembly comprises a worm wheel, a coil spring surrounding the outer peripheral edge of a cylindrical annular body. WO2004-027290A2 seeks to solve the problem in the previous application that a great amount of heat is generated by the friction between the coil spring and the annular body during the movement, and it applies a heat conducting unit to dissipate heat.
However, the coil spring in conventional linear actuator is fixed, and the cylindrical annular body rotates with the worm wheel, so that heat is generated by the friction between the coil spring and the cylindrical annular body. In addition, the cylindrical annular body is generally made of plastics, so that the heat would cause the cylindrical annular body melting or the coil spring being worn out rapidly. Thus, the lifespan is greatly decreased. Also, the braking effect is achieved merely by linear contact between the coil spring and the cylindrical annular body, which is rotating, for decelerating or braking. Since the effective contact areas between the coil spring and the annular body is small, the ability of decelerating or braking is limited.